1. Field of the Invention
The present invention relates to lighter-than-air aircraft and, more particularly, relates to an apparatus and method for launching a lighter-than-air aircraft having an envelope that can be substantially full during launch.
2. Description of Related Art
Lighter-than-air (LTA) aircraft, such as zeppelins, dirigibles, blimps, and balloons typically include an envelope or container that receives a gas that is lighter than air so that the aircraft is made buoyant. The buoyancy provided by the gas can be used to lift the vehicle to flying altitudes of 50,000 feet or higher, as is known for so called high altitude platforms (HAPs). For example, a conventional blimp includes a large envelope formed of non-rigid material that is inflated with helium. The helium provides sufficient buoyancy to lift the blimp to its flying altitude. Propulsion units and control devices such as fins, vanes, and the like provide power and control for adjusting the flight path and attitude of the blimp.
A conventional LTA aircraft is typically moored to a tall mast when not in flight. A nose cone, which is attached to the nose or bow of the aircraft by battens, provides a reinforced structure for connecting to the mast. The nose cone is rotatably connected to the mast so that the aircraft rotates freely around the mast under the force of the wind. The final assembly of the aircraft can be completed with the aircraft moored on the mast and the aircraft subsequently can be launched from and landed on the mast. During launching, the gas in the envelope and/or ballast on the aircraft can be adjusted so that the aircraft is slightly aerostatically heavy, i.e., non-buoyant. The aircraft is then disconnected from the mast, and maneuvering engines are used to propel the aircraft away from the mast. During landing, the aircraft is maneuvered back to the mast, and handling lines attached to the nose of the aircraft are dropped to a ground crew, which uses the lines to guide the aircraft to the mast so that the nose cone can be reattached thereto. Once moored, the aircraft can be refueled and several maintenance procedures can be conducted without bringing the aircraft into a hangar.
The conventional mooring operation is labor intensive and expensive. A large area must be provided around the mast for the movement of the aircraft. In addition, the design of the aircraft is influenced by the mooring operations. For example, the weight of the nose cone, battens, maneuvering engines, handling lines, and other equipment for near-ground operation, such as ground impact protection, increases the weight of the aircraft, and, hence, the necessary size and capacity of the gas-filled envelope. This equipment is typically used only during near-ground operations and is unused during the rest of the flight of the aircraft. Further, the configuration and materials of the envelope and the rest of the aircraft must be designed to accommodate the functions and stresses associated with the mooring operation.
LTA aircraft such as HAPs can also include a ballonet, i.e., an inflatable bladder, that is positioned within the envelope and configured to be expanded to nearly fill the envelope. The ballonet can be filled with air, and the space within the envelope that is outside the ballonet is filled with helium. As the aircraft ascends, the helium expands and the air is vented from the ballonet so that the size of the ballonet becomes increasingly smaller while the envelope remains at a substantially constant volume. As a result of the constant volume of the envelope, the aerodynamic and structural aspects of the airship remain mostly constant during flight. Depending on the position of the ballonet, the center of buoyancy of the aircraft can be adjusted during ascent so that the pitch or orientation of the aircraft changes. However, the ballonet adds weight to the aircraft. Additionally, the ballonet can slosh, or move unpredictably, in the envelope, affecting the structural integrity of the envelope and the orientation of the aircraft.
According to another method for launching a LTA aircraft, the envelope is only partially filled with helium so that the envelope is in a slack or limp condition. As the aircraft rises, the helium expands to fill the envelope. Advantageously, there is no need for a ballonet. Further, the aircraft can be launched from the ground without the use of a mast, similar to the launching of a weather balloon, thus simplifying the ground equipment necessary for launch. The weight of the aircraft can be reduced by including no maneuvering engines, nose cone, and the like. However, because the envelope is inflated only upon ascent, equipment that is connected to the envelope and positioned by the inflation of the envelope may not be properly positioned and adjustment in flight may be difficult or impossible. Some equipment, such as solar cells connected to the outside of the envelope, can be easily damaged when the envelope changes shape. Also, stresses on the envelope during the ascension while the envelope is only partially expanded are difficult to predict, and the envelope may be damaged during ascent due to flutter or aeroelastic effects, especially as the aircraft rises through winds, such as those associated with jet streams. Since the material that is used to form the envelope is typically only slightly flexible, wrinkling can damage the fibers and/or coatings of the envelope, causing pinholes, tears, weakened areas, and the like.
Thus, there exists a need for an improved apparatus and method for launching and recovering a LTA aircraft. Preferably, a large area should not be required for launching, and the method should minimize the required heavy equipment on the aircraft for near ground operation, such as nose cones, handling lines, maneuvering engines, and the like. Further, an aircraft launched according to the improved method should maintain a substantially constant shape during launch to minimize structural and aerodynamic changes.